Stem cells have both the capacity to self-renew, that is, to divide and create additional stem cells, and also to differentiate along a specified molecular pathway. Embryonic stem cells are very nearly totipotent, reserving the elite privileges of choosing among most if not all of the differentiation pathways that specify the animal. In contrast, stem cells that reside within an adult organ or tissue have more restricted options, often able to select a differentiation program from only a few...
